My research associates, postdoctoral fellows, and graduate students and I will conduct research along two principal lines: 1) photoreceptors and 2) cytodifferentiation. Under the former we shall continue to sample photoreceptors in animal phyla not yet examined by electron microscopy, further our efforts to determine the effects of light and darkness upon the ultrastructure of light-sensitive cells, continue our investigation of the function of the unique 800 A vesicles in the photoreceptoral cells of molluscan eyes, and study regeneration of ocelli in seastars. The chief thrust in embryology, principally by my graduate students, will be in the area of early ultrastructural differentiation in the neural tube of amphibian embryos with particular attention to cell death, the role of microfilaments in the embryonic muscle cells of the chick, and the differentiation of chick thyroid with special attention to the role of microtubules in the morphogenesis of the gland.